1. Field of Invention
The present invention relates to a transflective liquid crystal display (LCD). More particularly, the present invention relates to a single cell gap transflective liquid crystal display.
2. Description of Related Art
Due to progress in the development of semiconductor devices or man-machine display interface, multimedia communication is almost a routine in everyday life. High-quality and economical displaying devices such as cathode ray tube (CRT) are in the market for some time. However, as a desktop terminal/display, CRT is rather bulky and occupies a lot of space, and from the standpoint of energy conservation, CRT consumes too much electrical energy. Hence, CRT can no longer meet our demands for a light, compact and energy efficient display. Since the recently developed thin film transistor liquid crystal display (TFT-LCD) has superior image quality, slim, low power consumption and radiation free characteristics, TFT-LCD is now a major sell in the market.
Most liquid crystal displays (LCD) can be categorized into the transparent type, the reflective type and the transflective type. The classification is based on the different light source utilization and array arrangements. The transparent LCD uses back light as a source of illumination and the pixel electrodes on the array are transparent to facilitate the penetration of back light. The reflective LCD uses front light or external light as a source of illumination and the pixel electrons on the array are made from metal or other substances having good reflective properties so that the front or external light can be reflected. The transflective LCD uses both back light and external light as a source of illumination at the same time. Each pixel can be divided into a transparent area and a reflective area. The transparent area has a transparent electrode that facilitates the passage of back light and the reflective area has a reflective electrode capable of reflecting light from external light sources.
Using a normally black transflective LCD as an example, both the transparent area and the reflective area are in a dark state before the application of a voltage. When the transparent area and the reflective area change from a dark state to the brightest state, phase in the transparent area must differ by ±λ/2 and phase in the reflective area must differ by ±λ/4. However, in a single cell gap LCD, the required phase differences are hard to secure at the same time. Thus, optimal utilization of light in both the transparent area and the reflective area is difficult to attain in practice. Due to intrinsic display limitations of a single cell gap transflective LCD, transflective LCD having dual cell gaps are developed. By designing the transparent area and the reflective area with different cell gaps, light from whatever sources is fully utilized.
FIG. 1A is a schematic cross-sectional view of a conventional dual cell gap transflective liquid crystal display. As shown in FIG. 1A, the dual cell cap transflective LCD 100 mainly comprises of a thin film transistor (TFT) array substrate 102, a facing substrate 104 and a liquid crystal layer 106. The cell gap in the transmission region (T) of the transflective LCD is controlled to a distance d while the cell gap in the reflective area (R) of the transflective LCD is controlled to a distance d/2. Hence, the liquid crystal layer 106 within the transparent area (T) has a thickness d and the liquid crystal layer 106 within the reflective area (R) has a thickness d/2. In addition, the cell gap or the thickness d of the liquid crystal layer 106 must also meet the phase change relationship (Δn.d)=±λ/2. Therefore, through a thickness variation (d to d/2) of the liquid crystal layer 106, there is a phase change of ±λ/2 and ±λ/4 inside the respective cell gaps.
FIG. 1B is a schematic layout diagram of a conventional dual cell gap transflective LCD. As shown in FIG. 1B, the active device array substrate 102 has a plurality of scanning lines 200 and a plurality of data lines 202 thereon. Each pair of neighboring scanning lines 200 and each pair of neighboring data lines 202 constitute a pixel region 212. Each pixel region 212 has an active device 204, a transparent electrode 206 and a reflective electrode 208. The transparent electrode 206 is positioned over a portion of the pixel region 212 to form a transparent area (T). The reflective electrode 208 is positioned over a portion of the pixel region 212 outside the transparent area (T) to form a reflective area (R).
In general, the transparent electrode 206 and the reflective electrode 208 in the same pixel region 212 are electrically connected together. Hence, the transparent electrode 206 and the reflective electrode 208 within the same pixel region 212 are controlled by one active device 204. Furthermore, the active device 204 is, for example, a thin film transistor (TFT) or a diode that may switch state when driven voltages applied to the scanning line 200 and the data line 202.
Although the dual cell gap transflective LCD is able to optimize illumination, the substrate plates are difficult to fabricate.